The present invention relates to means for testing transmissive optical fibers, and more particularly to the high resolution measurement of Polarization Mode Dispersion (PMD) values in single mode optical fibers.
Single mode optical fibers are used to transmit large quantities of information over significant distances. In order to preserve the integrity of such transmissions, it is desirable to eliminate distortion. It is impossible, however, to remove all forms of distortion from transmissive media. Therefore, it is necessary to measure the distortion, either to determine the suitability of a transmissive medium maximum information capacity, or to determine the most satisfactory manner of handling the distortion. For a fiber optic communications system, the most significant specification for determining the information carrying capacity of the system is the bit-error rate. The bit-error rate is increased by, among other factors, the pulse broadening caused by dispersion in a fiber. Use of a single mode fiber eliminates modal dispersion, but not chromatic dispersion or polarization mode dispersion (PMD). PMD is a bandwidth limiting effect that is present to some degree in all single mode fibers that are suitable for optical transmissions. It is, therefore, a potential source of signal distortion in optical communications systems.
Current methods exist for measuring dispersion in optical fibers, for example, the system of U.S. Pat. No. 4,750,833 issued Jun. 14, 1988 to Roger S. Jones. Jones describes a method and apparatus for measuring transmissive dispersion, such as chromatic or polarization dispersion. A light source modulated at a first frequency is synchronously varied at a lower frequency back and forth to and from a first and a second value of a transmissive parameter, e.g. source wavelength or polarization state. Relative phases of the first modulation signal and the light transmitted through the fiber under test are measured by a phase detector. A lock-in amplifier compares the phase detector output to the lower frequency signal to provide a direct current output indicative of dispersion. This method and apparatus proved to be superior in resolution than a system measuring time differences. However, with progress in the art, a method and apparatus for measuring PMD with a resolution at least an order of magnitude higher than achievable with a relative phase or time measurement system are highly desirable. Indeed, the present invention contemplates measurement of PMD with approximately two orders of magnitude better resolution than that afforded by the above-described relative phase method.
Other current methods of measuring polarization mode dispersion in optical fibers, include Interferometry, Jones Matrix Eigenanalysis, the Wavelength Scanning (WS) cycle counting method, and the WS Fourier method. Interferometry uses the time domain to employ a low-coherence light source and a Michelson or Mach-Zehnder Interferometer to observe output in the form of the autocorrelation function of the time distribution, and the PMD of the fiber may be obtained from this data. Interferometry is limited at the low end by the coherence time, typically 0.15 ps, of the broadband source used.
Jones Matrix Eigenanalysis uses a polarimetric determination of the instantaneous polarization transmission behavior, in the form of a Jones matrix with two eigenstates called Principal States of Polarization (PSP). By measuring the wavelength variation of the Jones matrix and hence the PSPs, the differential delay between PSPs may be determined. This delay is averaged over a specific wavelength scan to establish the fiber PMD value. Jones Matrix Eigenanalysis is limited by polarimetric accuracy and resolution to 0.01 ps.
The WS cycle counting and the WS Fourier methods both use a light power transmission through the fiber using a linearly polarized source and a polarization analyzer before the light detector. The fiber gives rise to an oscillation pattern whose oscillation frequency is related to PMD. In the WS cycle counting method, the number of complete oscillations in a given wavelength interval is counted to determine PMD. The WS cycle counting method is limited to a minimum of three cycles in the wavelength scan, typically 0.09 ps. In the WS Fourier method, however, wavelength scanning PMD is determined by a frequency analysis technique on the oscillation pattern based on a Fourier transform. The Fourier method is limited to a minimum one cycle in the wavelength scan used, typically 0.03 ps.